U-Pb age and genesis of the Hejiazhuang-Laohuyao intrusion in West Qinling and limitation of plate subduction time

The Hejiazhuang-Laohuyao intrusions are located in the north of the Mianlue suture zone, close to the west section of the Shangdan fault zone in the north of the Qinling microplate, and their main lithology is granodiorite. The authors studied the LA-ICP-MS zircon U-Pb ages, geochemical characteristics and Hf isotope characteristics of the Hejiazhuang-Laohuyao intrusions. The results show that the emplacement time of the Hejiazhuang intrusion is 235-240 Ma and that of the Laohuyao intrusions is 239 Ma, suggesting that their emplacement epochs were the early Triassic. The granodiorite in the Hejiazhuang and Laohuyao intrusions are both high-potassium calcareous and alkaline series, showing strong over-aluminum characteristics and belonging to type I granite. The ε_Hf (t) values of both magmatic zircons are mostly negative and contain one or two positive values. The model age of the crust is Mesoproterozoic (1255-1754 Ma), indicating that their source rocks experienced the addition of mantle materials, which might have been magma mixed with mantle materials dominated by partial melting of Mesoproterozoic ancient crust, showing similar geochemical characteristics to adakianrocks and indicating that the magma might have originated from high potassium solution magma mixing produced by partial melting of subducted oceanic crust under high pressure and partial melting of lower crust contaminated by peridotite due to bottom transgression. The emplacement time and genesis of them indicate that the Yangtze plate and Qinling microplates were subducted during the period of 235-240 Ma.